1. Field of the Invention
The present invention relates to control of paper feed in an image forming system.
2. Description of the Related Art
An image forming apparatus to which multiple optional units can be connected has been proposed heretofore (see the specification of Japanese Patent Application Laid-Open No. 08-286567). This image forming apparatus controls each optional unit through an overall controller.
In order to increase the number of sheets of printing paper that can be fed, it is desired that a plurality of optional paper feeders be connected to the image forming apparatus in multiple stages. Such an image forming apparatus often employs serial communication in order to transmit a motor-stop command or a drive-start command to each paper feeder.
In a case where the above commands are transmitted to each paper feeder by serial communication, however, the traveling time of the command to each paper feeder deviates depending upon the distance from the image forming apparatus (see FIGS. 8A and 8B).
For example, traveling times of commands to each of the paper feeders connected to the image forming apparatus differ from one another. Consequently, the time between preceding and following sheets of printing paper varies depending upon the difference in traveling times. With such an arrangement, even if it is attempted to raise throughput by raising the speed of paper feed, a limitation is imposed because it is necessary to take into account the variance in time between the paper sheets. Further, owing to the difference in command traveling times, paper jamming may occur if the time between paper sheets becomes smaller than the appropriate value. For example, if a plurality of paper feeders is used for feeding paper sheets, the time between the preceding sheet and the following sheet varies at the timing of switching one paper feeder to another paper feeder. If the sheet-to-sheet space is prolonged due to the deviation between the traveling times of the command to each paper feeder, the throughput may go down. On the other hands, if the sheet-to-sheet space is shortened due to the deviation, the paper jam may be occurred. Further, the timing of stopping the paper sheet deviates in case that the paper sheet, which strides over the plurality of paper feeders, is instructed to stop. More particularly, the jam is occurred, since the paper sheet is pressed into the paper feeder located in downstream or is pulled by two paper feeders. In addition, image adjusting processing (e.g., processing for adjusting toner density and processing for adjusting paper registration) executed in the time between sheets can no longer be performed sufficiently. This is undesirable as it results in a decline in image quality.